This invention relates in general to Electrically-assisted Power Steering (EPS) systems for motor vehicles and in particular to an EPS system that also provides vehicle anti-theft protection.
In the past, vehicles have been equipped with Hydraulically-assisted Power Steering (HPS) systems. As is well known in the prior art, conventional HPS systems include a hydraulic actuator that is connected to the vehicle steering linkage. The hydraulic actuator is controlled by a rotary control valve that is connected to the vehicle steering wheel. A pump supplies pressurized hydraulic fluid to the rotary control valve which is operable to supply a portion of the hydraulic fluid to the actuator. When the vehicle operator turns the steering wheel, the rotary control valve is rotated. Rotation of the rotary control valve causes displacement of the hydraulic actuator which, in turn applies a directed force to the steering linkage to assist the vehicle operator in turning the steerable front vehicle wheels. In a HPS system, the pump is continuously driven by a belt from the vehicle engine crankshaft. Accordingly, HPS systems impose a continuous power requirement upon the vehicle engine.
Recently, Electrically-assisted Power Steering (EPS) systems have been developed to replace HPS systems. An EPS systems includes an electric assist motor that, upon actuation, applies torque to the vehicle steering linkage to assist the vehicle operator in turning the front vehicle wheels. Because the electric assist motor is only actuated when the steering wheel is moved, the power requirements for generating the assist torque are intermittent instead of continuous, as with HPS systems. As a result, operating of efficiency of the vehicle is improved. Also, the electrical wiring required  by the EPS system may be easier to route within the vehicle engine compartment that the hydraulic lines needed for a conventional HPS system.
Referring now to FIG. 1, there is shown a schematic drawing of a conventional EPS system 10. The system 10 includes a steering wheel 12 connected to an input shaft 14. The input shaft 14 is connected through a steering torque sensor 16 to an output shaft 18. The torque sensor 16 is operative to generate a torque requirement signal that is proportional to the torque applied to the steering wheel 12 by the vehicle operator. The torque sensor includes a torsion bar (not shown) that is connected between the input and output shafts 14 and 18. A position sensor (also not shown) also is connected to the input and output shafts. The position sensor senses the relative rotational position between the input and output shafts 14 and 18. Taking into account the torsional strength of the torsion bar, the sensed relative rotational position is indicative of the amount of steering torque applied to the steering wheel 12. The torque requirement signal is generated when the operator rotates the steering wheel and will decrease as the steerable wheels respond. Additionally, the torque sensor also will generate a torque requirement signal when the steering wheel is held stationary and the steerable wheels move in response to road surface conditions.
The output of the torque sensor 16 is connected to a steering Electronic Control Unit (ECU) 20 that includes a microprocessor (not shown) for controlling the EPS system 10. The microprocessor includes memory capacity, such as internal ROM and/or RAM, for storing an algorithm for controlling the operation of the EPS system 10. A vehicle speed sensor 21 provides a vehicle speed signal to the steering ECU 20.
Also shown in FIG. 1 is a conventional mechanical lock 22 that is mounted upon the steering input shaft 14. The lock 22 is normally actuated to prevent movement of the steering wheel as a vehicle theft deterrent. The lock 22 is electrically connected a body control module 24 that provides overall control functions over various vehicle systems, including the EPS system 10 and an engine control unit 25. The body control module 24 is connected to a key lock 26. In a known manner, insertion and rotation of a key in the key lock 26 causes the body control module 24 to deactuate the mechanical lock 22, allowing rotation of the steering wheel 12. 
The steering output shaft 18 is connected to a pinion gear (not shown) of a rack and pinion gear set 30. The rack and pinion gear set 30 functions to transform the rotational motion of the steering wheel 12 into linear motion of a steering rack 32. The steering rack 32 is connected to steerable vehicle wheels 34 in a conventional manner. The linear movement of the steering rack 32 deflects the wheels 34 to the right or left.
The EPS system 10 includes an electric assist motor 36. Typically, the assist motor 36 is operatively connected to the steering rack 32 through a ball nut assembly (not shown) in a conventional manner. Alternately, the assist motor 36 can be coupled to column drive systems, pinion drive systems, or other conventional steering systems. When the electric assist motor 36 is energized, the motor rotor turns, which, in turn, rotates the nut portion of the ball nut assembly. When the nut portion rotates, the balls transfer a linear force to the steering rack 32, thereby providing an assistance torque to aid the driver in turning the wheels 34. The direction of movement of the steering rack 32 is dependent upon the direction of rotation of the electric assist motor 36. A motor rotor position sensor 38 is mounted upon the motor 36 and is connected to the steering ECU 20. One of the functions of the motor rotor position sensor 38 is to provide an electrical signal indicative of the position of the motor rotor relative to the motor stator to the steering ECU 20. For proper operation of the electric assist motor 36, including direction of rotation and applied torque, it is necessary to know the position of the rotor relative to the stator. Optionally, the motor rotor sensor 38 also may provide additional information concerning current flow through the motor 36.
A motor drive circuit 40 is connected to and actuates the electric assist motor 36. Electric power is supplied to the drive circuit 40 through a power supply relay 42 by the vehicle power supply 44. Both the drive circuit 40 and power supply relay 42 are connected to the steering ECU 20 which is operational to control both devices.
A schematic drawing of the motor drive circuit 40 is shown in FIG. 2. Components shown in FIG. 2 that are similar to components shown in FIG. 1 have the same numerical designators. As shown in FIG. 2, the electric assist motor 36 is a multi-phase brushless star connected permanent magnet motor. The motor 36 includes  three motor windings that are labeled R, Y and B. Each of the motor windings R, Y and B has a first end and a second end. The first ends of the motor windings are connected to the motor drive circuit 40. Specifically, the first end of each motor winding is connected between a corresponding pair of electronic switches, which are shown as bipolar transistors in FIG. 2 but could be other devices, such as, for example FET's. A first transistor in each pair, labeled T1B, T1Y and T1R for the corresponding winding, is connected between each phase winding and the power supply relay 42 while a second transistor in each pair, labeled T2B, T2Y and T2R for the corresponding winding, is connected between each winding and ground. The bases of each of the transistors are connected to the steering ECU 20.
The second ends of each of the motor windings R, Y and B are connected to motor winding relay 43. As shown in FIG. 2, the motor winding relay 43 has two sets of contacts each of which separates the second end of one of a pair of the motor windings B and R from the second end of the third winding Y. When the relay contacts are closed, as during normal operation of the motor 36 and motor drive circuit 40, the second ends of all three motor windings are connected to form the star connection. The contacts in the motor winding relay 43 are opened if a fault is detected in the EPS system 10, such as, for example, a shorted motor winding or transistor in the motor drive circuit 40. Opening of the motor winding relay contacts isolates each of the motor windings from the other of the motor windings and from the winding that is common to the relay contacts. This prevents back EMF generated by movement of the steering column (and hence the motor rotor) from finding a low impedance path through the control circuit and thus prevents electrical braking of the steering apparatus.
The microprocessor in the steering ECU 20 controls the amount of steering assist provided by the motor 36 as a function of both the applied steering torque and the vehicle speed. The microprocessor is responsive to signals received from the steering torque sensor 16, the motor rotor position sensor 38 and the vehicle speed sensor 21 to generate steering command signals. The microprocessor can also receive data from other vehicle systems, such as for example, the temperature, that it can  utilize in generating the steering command signals. The steering command signals are applied to the transistors in the motor drive circuit 40 to selectively energize the motor phase windings B, Y and R. As a result of energizing the motor phase windings, the motor rotor will rotate in a desired direction to provide an assist torque to the steerable wheels 34. For example, switching transistors T1B, T1R and T2Y to their conducting state will cause a current to flow through the motor windings to energize phase Y and to produce a corresponding motor torque. Similarly, switching transistors T1Y, T2B T2R to their conducting states will result in an opposite current flowing through the motor windings to energize phase Y in the opposite direction and to produce a counter-torque.
As shown in FIG. 1, the EPS system 10 includes a mechanical lock 22 as an anti-theft device. It would be desirable to utilize the EPS system 10 to provide theft deterrence and thereby eliminate the need for a mechanical anti-theft device. An EPS system that incorporates an ant-theft feature would simplify the steering system by reducing the number of components with a corresponding reduction in cost.